Grinding machine

ABSTRACT

This invention relates to a grinding machine and, more particularly, to a system for preventing thermal damage in abrasive machining and finishing by measuring abrasive wheel dullness and providing for a renewal of the wheel surface when the dullness reaches a preselected value.

United States Patent [1 1 Hahn et al.

[ 1 July 17, 1973 1 GRINDING MACHINE [75] lnventors: Robert S. Hahn, Northboro; Richard P. Lindsay, Marlboro; Edward G. Robillard, Cherry Valley; David H. Youden, Worcester, all of Mass.

[73] Assignee: The Heald Machine Company,

Worcester, Mass.

221 Filed: Feb. 1, 1971 211 Appl.No.: 111,608

A Related US. Application Data [63] Continuation of Ser. No. 717,853, April 1, 1968,

abandoned.

[52] US. Cl. 51/5, 51/l65.87 [51] Int. Cl. B24b53/00 [58] Field of Search 51/5, 165 R, 165.87;

MAIN CONTROL BENTLEY GAGE [56] References Cited UNITED STATES PATENTS 2,778,170 l/1957 Flanders 51/165.87 X 3,125,087 3/1964 Johnson.... 125/11 R 3,167,891 2/1965 Coes et al..... 5l/l65.71 3,327,432 6/1967 Lockwood 5l/l65.87

Primary Examiner-Harold D. Whitehead Attorney-N0rman Blodgett [57] ABSTRACT This invention relates to a grinding machine and, more particularly, to a system for preventing thermal damage in abrasive machining and finishing by measuring abrasive wheel dullness and providing for a renewal of the wheel surface when the dullness reaches a preselected value.

10 Claims, 11 Drawing Figures PAIENIEDJUUTIHYS sum 1 or 6 MAIN CONTROL 90- BENTLEY GAGE INVENTORS ROBERT S. HAHN RICHARD F. LINDSAY EDWARD G. ROBILLARD DAVID H. YOUDEN PAIENIEU JUL 1 71m SHEET 2 OF 6 REAL AREA OF CONTACT A =.O0O67'6 IN REAL AREA OF CONTACT A O003|2 IN REAL AREA OF CONTACT A =.oo43 IN2 DULL m 0 O T 5 Y w W m 2 AT .L w w m Am E F LW G AC W %F L O O P O S ,0 a mm m S 0 E V OE M v 2M n M W E u I Qxw w w... o 832: $63.; H 623. .0 Q P u u fl n n a I M A 0 0 o w L xm w L fzl 5528 Lo 5% 25 PAIENIED JUL 1 7191s SHEET [1F 6 THEKQETICAL AEXPERIMENTAL VS.

FOR 80 GRIT TESTS F =7.75,l5,3OL8.

DRESSING CIRCUIT 53 TRTGGER n: A M E FEED PRESSURE 45\TRANSDUCER p VEL.

X SLIDE VELOCITY v TRANSDUCER PATENIEBJUL 1 71m SHEEV 5 BF 6 FEED PRESSURE TRANSDUCER v r I" 6 I FM 4 e;

T|MER- 4| 45 1 e O VEL. V

X SLIDE VELOCITY TRANSDUCER HOLDING COIL FIG] FORCE PULSER TO CROSS SLIDE SERVO HYDACTUATOR VALVE 8, LO e.

1, J 6 K PRESSURE G g X K TACHO GENERATOR FORCE TRANSDUCER L 97 TIME SPEED PULSER GRINDING MACHINE This application is a continiuation of Ser. No. 717,853, filed Apr. 1, 1968, now abandoned.

BACKGROUND OF THE INVENTION In the manufacture of certain machine parts, such as jet engine parts and anti-friction bearing raceways, a problem often arises known as thermal damage. This usually consists in loss in metallurgical properties and is generally understood to be due to heavy feed rates or force intensities. However, thermal damage can also be caused even with finish grinding forces when the grinding wheel is dull. Consequently, thermal damage is related to two important factors: force intensity and wheel dullness. It is difficult to operate an automatic grinding machine at its optimum efficiency without exceeding the permissible limits of the above factors and causing thermal damage. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a grinding machine capable of operating at maximum efficiency without causing thermal damag to the workpiece.

Another object of this invention is the provision of a grinding machine incorporating a means for measuring wheel dullness and using it to regulate the machine operation.

A further object of the present invention is the provision of a grinding machine having an operating cycle using a measurement of wheel dullness to regulate wheel renewal.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

SUMMARY OF THE INVENTION In general, the present invention has to do with a grinding machine having a base, having a workhead mounted on the base to support a workpiece having a surface of revolution to be finished by the abrasive process, having a wheelhead also mounted on the base and having a rotatable spindle carrying an abrasive wheel, having feed apparatus to bring about relative movement between the workhead and wheelhead to introduce the wheel to the workpiece for performing a grinding operation to finish the said surface of revolution, and having means operative when wheel dullness exceeds a predetermined value to bring about renewal of the wheel surface.

More specifically, the feed apparatus produces the said movement with either controlled force or controlled rate and the said means continuously monitors the relationship of the grinding force to the velocity of the said movement and the renewal of the wheel surface takes place by dressing or by a sudden increase in wheel pressure. Alternatively, the means monitors the relationship between the grinding force normal to the surface of revolution and the tangential force.

BRIEF DESCRIPTION OF THE DRAWINGS The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings, in which:

FIG. 1 is a plan, somewhat schematic view, of a grinding machine incorporating the principles of the present invention, f

FIG. 2 is a chart showing the concept of real area of contact in an abrasive wheel,

FIG. 3 is a graph showing the relationship between real area of contact and plunge velocity,

FIG. 4 is a graph showing the relationship between force intensity and penetration rate,

FIG. 5 is a graph showing the relationship between the metal removal parameter and real area of contact,

FIGS. 6, 7, and 8 are schematic diagrams showing methods of operating the grinding machine,

FIG. 9 is a graph showing the relationship between wheel sharpness and the ratio of tangential to normal grinding force, and

FIGS. 10 and 11 are schematic diagrams showing still further methods of operating the machine.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, wherein are best shown the general features of the invention, the grinding machine, indicated generally by the reference numeral 10, is shown as consisting of a base 1 1 having a flat horizontal upper surface on which are mounted a workhead 12 and a wheelhead 13. The workhead is carried on a workhead table 14 which is mounted for longitudinal motion on the base by means of ways 15. Similarly, the wheelhead 13 is carried on a wheelhead table 16 which is mounted for transverse sliding motion on the base 1 l by means of ways 17. Extending between the workhead table 16 and the base 11 is a feed cylinder 18 which is suitably provided with fluid pressure to bring about movement of the table 16 transversely of the base. Similarly, a hydraulic cylinder 19 connects the table 14 to the base 11 and brings about longitudinal movement of the table 14 over the ways 15. Extending from the wheelhead 13 is a rotatable spindler 21 carrying on its outer end an abrasive wheel 22. Carried by the workhead 12 and rotated thereby is a workpiece 23; for the purpose of illustration, this workpiece is shown as the outer race of a ball bearing whose inner groove is being ground by the abrasive wheel 22. The workpiece is supported on its outer cylindrical surface by shoes in the usual way and rotatably driven by the workhead l2. Residing in the bore of the workpiece during the grinding operation is a pneumatic air gage 24.

Slidable on the upper surface of the base 1 l is a block 26 which, at its rearward end, is threadedly attached to a threaded shaft 27 which, in turn, is driven by a motor 28 fastened to the base 11. This motor is of the type capable of small discrete rotary increments in response to pulses received from an electrical control. By regulating the number and mode of the pulses from the electrical control, it is possible to rotate the motor quickly or slowly in exact amounts in either direction to move the block 26 transversely over the surface of the base 11. In the present apparatus, this motor is used to produce a compensation after dress and to provide for a retraction of the wheel at a suitable time in the grinding cycle. The dressing apparatus 29 is mounted on the table 14. The block 26 is provided with a transverse slot 31 and in this slot resides a horn 32 extending downwardly from the undersurface of the wheelhead table 16. The front of the block is provided with a hardened metal insert 33. Mounted on the front portion of the wheelhead table 16 is a retractable stop 34 having a contact rod 35 extending toward the block 26 and toward the metal insert 33. Mounted at the front of the retractable stop is a gear 36 and, to the right, is an adjusting knob 37. This retractable stop apparatus for producing feed rate control on occasion is similar to that shown and described in the patent application of Uhtenwoldt et al., Ser. No. 796,291 which is a continuation of patent application Ser. No. 572,930, filed Aug. 17, 1966, now abandoned.

Extending rearwardly to the rear of the table 16 is the core 38 of a velocity transducer having a coil 39 connected to an amplifier 41. The main pressure fluid is provided to the cylinder 18 through a conduit 42; a supplementary pressure pulse can be provided for a crush renewal of wheel surface through a branch conduit 43 by the operation of a valve 44 operated by a coil 45. Extending from the main conduit 42 is a transducer 46 giving an electrical output signal indicative of the pressure being supplied to the feed cylinder. The wheelhead 12 is driven by a motor 47 by a belt 48. The bight of the belt passes over an idler 49 which is supplied with air to support the belt in the manner shown and described in the patent of Youden No. 3,338,106. The idler is provided with pressure air through a conduit 51. Connected to the conduit is a transducer 52 capable of giving an electrical signal indicative of the air pressure in the conduit which, in turn, is indicative of the torque transmitted at any given time through the belt to the wheelhead.

Wheel dullness can be measured by the real area of contact," A,., as illustrated in FIG. 2. As the abrasive grains wear to produce larger wear flats, the grinding wheel produces more heat; thermal damage of the workpiece tends to occur. Also, the rate of cutting, i.e., the plunge grinding velocity, drops, as illustrated in the graph at the lower left corner of FIG. 2. The curves of FIG. 3 show the decrease of plunge velocity with time as the real area of contact increases. It can be seen, then, that the plunge velocity under a known force is a measure of wheel dullness. FIG. 4 clarifies this concept further. The upper curve shows the relationship between plunge velocity and force intensity for a sharp wheel, while the lower curve shows the same relationship when a dull wheel is used. The slope of these curves is referred to as the metal removal parameter, A; therefore, by measuring A, one can obtain a good indication of wheel dullness. The relationship between A and the real area of contact A, is shown in FIG. 5.

FIG. 6 shows a control system for maintaining thermal damage below a specified limit. This can be accomplished by monitoring the plunge grinding velocity and the corresponding normal force. Operating the grinding machine 10 as a controlled force machine, the normal wheel force results in an electrical signal F,,, from the transducer 46. The plunge velocity V emerges from the transducer 41. The ratio of the two signals is derived in a suitable circuit 53, the output signal being indicative of the metal removal parameters A. This signal is compared in a trigger circuit 54 with a predetermined signal A and, if it falls below that base value, current passes through a coil 55 associated with the dressing apparatus 29, so that the grinding operation is interrupted and the wheel removed from the workpiece for a surface renewal by dressing. The renewal can also take place, as shown in FIG. 7, by causing the electrical current to pass through the coil 45 to open the valve 44 for a short time to cause a pulse of high pressure fluid to pass into the feed cylinder 18. This causes a sharp increase in wheel force against the surface of the workpiece and causes a breakdown of the wheel surface, so that sharp new edges of the abrasive are exposed. The renewal of the wheel is similar to that produced by diamond dressing.

As shown in FIG. 8, a similar system can be used to provide optimum operation of the grinding machine with control of feed rate by use of the apparatus 34 without thermal damage. The wheelhead 13 is provided with apparatus such as that shown in the copending patent application of Hahn, Ser. No. 710,363 filed Mar. 4, 1968 in which the actual force between the wheel and the workpiece is monitored by a Bentley proximity pickup adjacent the spindle and is emitted from an amplifier 56. This signal is continuously compared in the circuit 57 with the signal V, from the circuit 41 and the resulting signal indicative of A is compared with a base value A If the sharpness of the wheel drops below this value, the trigger operates to renew the wheel surface, either by the speed pulser apparatus shown or by a diamond dress. In this particular arrangement, the feed rate system is combined with a closed-loop force control and, of course, with the A control which is the subject of the present invention.

Another method for detecting wheel dulling is through changes in the tangential grinding force. FIG. 9 shows how the ratio 1. of tangential force to normal force changes as the sharpness of the wheel changes. In this graph, the sharpness of the grinding wheel was defined as the volume of metal removed per unit of area of abrasive swept by under unit force intensity. It can be seen that the ratio drops from about 0.5 to 0.3 as the wheel becomes dull. In FIG. 10, the grinding machine is operated by the controlled force method, so that the normal force signal is obtained from the transducer 46. The tangential force is obtained from the transducer 52 which, it will be recalled, operates on the fact that the air back pressure is indicative of tension in the belt 48 as it passes over the idler 49. The tangential force could also be obtained by measuring the current flowing in one of the three-phase electrical lines supplying the motor 47, as is well-known, by means of a meter 91. Such an arrangement is shown and described in the patent of Dunigan No. 2,722,648 which issued on Nov. 1, 1955. The ratio u of the normal force F, to the tangential force F is compared in the circuit 58 and compared with a predetermined ratio ;1.,,. When p. drops to a value less than ;.t,,, the trigger circuit operates and energizes a coil which renews the wheel surface. The system shown in FIG. 11 operates in a similar manner, but the grinding machine is operated with control of feed rate instead of control of feed force. The derivation of the signal corresponding to the normal force must, therefore, come from the amplifier 56.

It can be seen, then, that the present invention provides an apparatus for grinding with the greatest possible efficicncy, whether with controlled feed force or controlled feed rate, without danger of thermal damage. It is possible to do this with simple and inexpensive apparatus entirely within the capability of present-day manufacturing and detection techniques.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

l. A grinding machine, comprising:

a. a base,

b. a workhead mounted on the base to support a workpiece having a surface to be finished by the abrasive process,

0. a wheelhead mounted on the base and having a rotatable spindle carrying an abrasive wheel,

d. feed apparatus to bring about relative movement between the workhead and wheelhead to introduce the wheel to the workpiece for producing a grinding force normal to the said workpiece surface and performing a grinding operation to finish the said surface, and

e. control means monitoring a metal removal parameter involving the grinding force normal to the surface and is operative as a function of wheel dullness to bring about renewal of the wheel.

2. A grinding machine as recited in claim 1, wherein the feed apparatus produces the said movement with controlled force and wherein the said means monitors the relationship of the grinding force to the velocity of the said movement.

3. A grinding machine as recited in claim 1, wherein the feed apparatus produces the said movement with controlled feed rate and wherein the said means monitors the grinding force and compares it with feed rate.

4. A grinding machine as recited in claim 1, wherein a dressing mechanism operates to renew the wheel surface.

5. A grinding machine as recited in claim 1, wherein a sudden increase in force between the wheel and the workpiece is used to renew the wheel surface.

6. A grinding machine as recited in claim 1, wherein the workhead is driven by a motor and a belt and wherein a measurement of tangential force with respect to the surface is obtained from the tension in the belt.

7. A grinding machine as recited in claim I, wherein the workhead is driven by a motor supplied with current through a three-phase system and wherein a measurement of the tangential force is obtained by measuring the current in one phase.

8. A grinding machine as recited in claim 1, wherein the normal grinding force is obtained by use of a proximity pickup located in the wheelhead adjacent the spindle.

9. A grinding machine as recited in claim 1, wherein the metal removal parameter A V /F, where V,,, is the velocity of recession of the workpiece surface and F, is the force between the wheel and the workpiece normal to the workpiece surface.

10. A grinding machine, comprising:

a. a base,

b. a workhead mounted on the base to support a workpiece having a surface to be finished by the abrasive process,

c. a wheelhead mounted on the base and having a rotatable spindle carrying an abrasive wheel,

d. feed apparatus to bring about relative movement between the workhead and wheelhead to introduce the wheel to the workpiece with controlled force for performing a grinding operation to finish the said surface, and

e. control means monitoring the relationship between a grinding force normal to the surface of a tangential force and is operative as a function of wheel dullness to bring about renewal of the wheel. 

1. A grinding machine, comprising: a. a base, b. a workhead mounted on the base to support a workpiece having a surface to be finished by the abrasive process, c. a wheelhead mounted on the base and having a rotatable spindle carrying an abrasive wheel, d. feed apparatus to bring about relative movement between the workhead and wheelhead to introduce the wheel to the workpiece for producing a grinding force normal to the said workpiece surface and performing a grinding operation to finish the said surface, and e. control means monitoring a metal removal parameter involving the grinding force normal to the surface and is operative as a function of wheel dullness to bring about renewal of the wheel.
 2. A grinding machine as recited in claim 1, wherein the feed apparatus produces the said movement with controlled force and wherein the said means monitors the relationship of the grinding force to the velocity of the said movement.
 3. A grinding machine as recited in claim 1, wherein the feed apparatus produces the said movement with controlled feed rate and wherein the said means monitors the grinding force and compares it with feed rate.
 4. A grinding machine as recited in claim 1, wherein a dressing mechanism operates to renew the wheel surface.
 5. A grinding machine as recited in claim 1, wherein a sudden increase in force between the wheel and the workpiece is used to renew the wheel surface.
 6. A grinding machine as recited in claim 1, wherein the workhead is driven by a motor and a belt and wherein a measurement of tangential force with respect to the surface is obtained from the tension in the belt.
 7. A grinding machine as recited in claim 1, wherein the workhead is driven by a motor supplied with current through a three-phase system and wherein a measurement of the tangential force is obtained by measuring the current in one phase.
 8. A grinding machine as recited in claim 1, wherein the normal grinding force is obtained by use of a proximity pickup located in the wheelhead adjacent the spindle.
 9. A grinding machine as recited in claim 1, wherein the metal removal parameter Lambda Vw/Fn, where Vw is the velocity of recession of the workpiece surface and Fn is the force between the wheel and the workpiece normal to the workpiece surface.
 10. A grinding machine, comprising: a. a base, b. a workhead mounted on the base to support a workpiece having a surface to be finished by the abrasive process, c. a wheelhead mounted on the base and having a rotatable spindle carrying an abrasive wheel, d. feed apparatus to bring about relative movement between the workhead and wheelhead to introduce the wheel to the workpiece with controlled force for performing a grinding operation to finish the said surface, and e. control means monitoring the relationship between a grinding force normal to the surface of a tangential force and is operative as a function of wheel dullness to bring about renewal of the wheel. 